o_1977r8vv9vk1ts2ms0kd8pksa.pdf

26 D. Maulik
far field the beam progressively diverges. In this circumstance,
the angle of beam divergence, which depends
on the wavelength and the transducer radius,
dictates the transverse dimension of the DSV. Obviously,
the width of the DSV is better defined in the
near field than in the far field. Consequently, operating
in the near field ensures greater certainty of velocity assessment.
In contrast to the sample length, the sample
width is not amenable to ready manipulation. However,
owing to progress in transducer technology, including
acoustic focusing, a few instruments allow controlled
variations of the beam width in the far field and therefore
of the transverse dimension of the DSV.
The dimensions of the sample volume can significantly
affect the accuracy of velocity assessment. A
sample volume that is large in relation to the dimensions
of the target vessel can significantly compromise
the range resolution of the measured velocities and
lower the signal-to-noise ratio [11]. Conversely, a sample
size smaller than the vascular dimension can diminish
the sensitivity of the system. Moreover, depending
on its relative size and location in the lumen of a large
vessel, the assessed velocity profile can be significantly
distorted. This subject is further discussed in Chap. 4.
Artifacts of Spectral Doppler
Sonography
Limitations of Fourier-based spectral analysis have
been described in a previous section of this chapter.
There are additional device-dependent false representations
of the Doppler information that may compromise
interpretive clarity. These artifacts are discussed
herein.
Nyquist Limit
A major problem of the PW Doppler system is its inherent
limitation in accurately representing Doppler
shifts from circulations with high-speed flow. A PW
Doppler is essentially a sampling tool for a cyclic
phenomenon. The sampling rate is the pulse repetition
frequency of the Doppler system; the cyclic phenomenon
is the Doppler shift frequencies generated
by blood flow. The relation between the pulse repetition
frequency and appropriate measurement of the
maximum frequency shift is dictated by Shannon's
theorem on sampling, which states that for unambiguous
measurement of a periodic phenomenon the
maximum frequency must not exceed half the sampling
rate [12], which amounts to a minimum of two
samples per cycle of the highest frequency. This maximum
frequency threshold is known as the Nyquist
frequency or Nyquist limit. The relation is expressed
in the following equation:
f max ˆ f pr =2
where f max is the maximum frequency measurable
without ambiguity (Nyquist limit), and f pr is the sampling
rate or the pulse repetition frequency.
Ambiguity in frequency resolution occurs when
the PW Doppler beam interrogates a vessel with such
a high blood flow velocity that the maximum frequency
content of the Doppler signal exceeds the Nyquist
limit. In this circumstance, the frequency cannot
be estimated precisely. The frequency component
in excess of the limit is subtracted from the Nyquist
frequency and is negatively expressed (Fig. 3.10). This
phenomenon is known as the Nyquist effect. It is also
Fig. 3.10. Example of aliasing
and its correction. Top Aliased
waveforms. Note the abrupt
termination of the peak portion
of the waveforms and the display
of these peaks in the lower
part of the panel below the
baseline (horizontal arowheads).
Bottom: Correction of aliasing
of the waveforms by increased
pulse repetition frequency